In a conventional seat with a heating device, as is shown in FIG. 13, heating elements 14a are installed individually in a seat portion 102 and a backrest portion 103 of a seat 101, so as to allow a user to be seated comfortably in the seat 101 even in a cold season. In addition, there has been a certain type of seat with a heating device in which a heating element 104b is installed even in a side (an inner side 106) of an armrest portion 105 of the seat 101 which faces a user so as to warm a side of the body of the user as well to thereby realize an increase in comfortableness felt by the user (for example, refer to Patent Document 1). Patent Document 1: JP-A-55-160522